Method of strengthening iron ore agglomerates



Sept. 17, 1957 J. H. VEALE ETAL METHOD OF STRENGTHENING IRON OREAGGLOMERATES Filed Sept. 16, 1954 United States Patent METHOD OFSTRENGTHENING IRON ORE AGGLOMERATES John H. Veale and Howard F. West,Coal City, 111., as-

signors to Illinois Clay Products Company, ioiiet, iii., a corporationof Illinois Application September 16, 1954, Serial No. 456,584 12Claims. (Cl. 75-5) This invention relates to a method of strengtheningiron ore agglomerates. More particularly, it relates to treating thesurface of taconite nodules during the final stage of their formationwith a composition which forms with the nodule a eutectic which wil fuseinto a strong bond and at a temperature just below the high temperatureof the present nodulizing proces. The invention may be adapted tohardening the surface of taconite pellets and to any other agglomeratein which there are substantial quantities of iron oxides, includinghematite (FezOs), on the surface of the agglomerate.

Applicants process is particularly adapted to the nodulizing proceswhich has been developed at the Mesabi Range to agglomerate taconiteiron ore because it can be inserted as a step in that process withoutaltering the kiln and with practically no capital investment. However,as above indicated, it can be adapted to sintering, pelletizing andbriquetting processes.

In nodulizing, a fine iron ore usually containing a binder or a flux orboth is moving through a rotating kiln in a gradually rising larger unitor agglomerate about the size of a walnut. As the nodules reach thedesired size, they move past the burner nozzles into a cooling zone. Thenodules are not of satisfactory strength, however. Between the productbin of the above-described process and the blast furnace, the nodulesreceive the same rough treatment that has always been given a hematiteore. They are moved by conveyors, picked up and dropped by shovels, andpoured into piles from spouts or feet high. A high percentage of thenodules break, and the more they break, the more further handling tendsto pulverize them. Taconite fines are useful in neither the blastfurnace nor the open hearth. In the former, a substantial percentage isblown into the dust catcher, and in the latter, the fines will not sinkthrough the slag.

It is essential that this defect in strength be cured for the taconitepellet or nodule has already proved its worth in competition with goodhematite. Having more iron than the hematite and containing less oxygen,F3304, (in taconite) against FezOa (in hematite), less flux and lesscoke is required to reduce the iron oxide in the taconite than in thehematite. With coke breeze at twenty dollars a ton, it is readily seenthat the use of taconite fines (their unavoidable condition at the endof the magnetic separation process) must not fail because of theinability to satisfactorily agglomerate.

The object of this invention is to strengthen the nodules, and othertaconite agglomerates, by giving them a stronger surface. Thecomposition of this improved surface must not be detrimental to blastfurnace operation. Advantageously, the surface should be placed upon thenodule in the course of the agglomerating process. All of theseore-agglomerating processes increase the cost of the blast furnace rawmaterial. At the present time, the additional cost of these processesdoes not impair the competitive character of the end product, firstly,be-

temperature, and snowballs into a ice cause the competition of cheaperores, such as hematite, is partly offset by additional transportationcharges (from more distant fields), and the taconite nodules aresubstantially higher in iron content. Nevertheless, each additional stepis costly and is practical only if it can be worked into an existingprocess without substantial additional cost.

The following is a detailed description of the invention furnished inconjunction with the drawings in which:

Fig. 1 is a diagrammatic view in side elevation of a form of apparatusparticularly suitable for carrying out the present process, a portion ofthe Wall of the apparatus being broken away to better illustrate theinvention;

Fig. 2 is an enlarged sectional view taken on the line 2-2 of Fig. 1looking in the direction of the arrows; and

Fig. 3 is an enlarged view showing the fuel and airmixing portion of theapparatus of Fig. l and illustrating the manner in which theagglomerate-strengthening agent may be introduced into the apparatus inaccordance with a preferred embodiment of the invention.

Referring to Fig. l, applicants schematically illustrate a rotary kiln10 as presently used in the nodulizing process at one plant in theMesabi Range. This kiln is 350 feet long, 11 /2 feet in diameter, and isinclined at a slope of one-half inch per foot. It rests on piers such as12, and is rotated by means not shown. The kiln is lined with refractorybrick 14, the thickness of which increases toward the delivery end 16.Extending inwardly of the delivery end of the kiln are burners.Applicants show a main burner 18 which is designed to direct a lazyflame 20 having a length of 50 to feet axially of the kiln. Disposedbeneath the burner 18 is an auxiliary burner 22 which is directeddownwardly toward the wall of the kiln. The nozzle of this burner andthe air pressure supplied to it produce a hard flame.

A charging chute 24 is disposed at the feed end 26 of the kiln and thecharge moves downwardly along the dotted line 28, immediately under thehard flame burner 22, and ultimately through a cooling unit 30 to ahopper 32. Referring to Fig. 2, the kiln 10 is rotating clockwise sothat the charge tends to collect as illustrated between the dotted line34 and the wall of he kiln. The particles tumble over one another andgrow in size. The hard flame burner 22 is directed at an angle into thismass of tumbling nodules. The kiln is operated with a charge content ofbetween 10 to 15 percent of its gross volume.

In operation, the nodules may be started by mixing seeds into the chargeof taconite fines from the bin 24. These seeds are particles that aresubstantially larger than the fines. The temperature of the hot gasesleaving the feed end 26 of the kiln will be in the neighborhood of 500F. In the course of the movement of the nodules down the kiln, the hotgases will heat the growing nodules. The rate of rotation of the kiln,the weight of charge fed to the kiln per unit of time, and the slope ofthe kiln are such that a nodule of a desired size will be obtained atthe time of the sharp final heating under the hard flame 22. It will beappreciated that skill in operation is of the utmost importance and thatthere is a considerable range in which the kiln's rotation and amount ofcharge introduced per unit of time may be varied, usually to com pensatefor variations in the chemical constitution of the charge. In thenorthern part of the Mesabi Range, the iron in the taconite is almostall magnetite, that is, FesO-t. In the southern part of the field, theiron in the taconite is almost wholly hematite, that is, FezOs. Thehematite is non-magnetic and the magnetite is magnetic, and since all ofthe processes presently developed rely upon magnetic separation, thepresent development in the Mesabi Range is taking place at the northernend of the field where primarily magnetite is encountered. For thepurposes of this invention, the magnetic quality of the iron compound isnot important.

The foregoing description of the nodulizing process is to make clearthat despite variations in technique and the chemical compositions inthe charges, nodules reach a desired size at a point in the kilnimmediately in advance of the burners. There is a space at this pointfor performing a surface operation on the nodules.

The first important feature in applicants process there fore is toperform the surface-solidifying step at a point where furtherenlargement of the nodules is not desired. This occurs substantiallyadjacent the hard flame nozzle 22. it is at this point that thetemperature of the nodules no longer continues to rise. it is true thatalter the nodules have passed under this bur" lr turc such,approximately 2430 F, t hliug. which occurs, may slightly increase tutualinnetcr. However. this is unavoidable unless the llOLililU o r droppedimmediately into a cooler, which also is undo sirable.

The second important step in applicants process is the selection of thebonds which will strengthen the surface of the ngglomerates. The bondswere initially sou ht for and selected because the temperature at itshottest point in the nodulizing process is in the neighborhood of u F.Applicants recognized that if a bond could be found which formed belowthis temperature and which was nevertheless strong bond, a glaze orpartial glaze could be put on the nodules. In U. S. Letters Patent2,783.44)". dated March 8. 1955, the applicant vcalc disclosed u processfor hardening the surface of briquettes containing a large amount ofsilica by means of depositing on the surface of the briquettes an oxideof calcium, or mag ncsium, or of calcium and magnesium. The temperatureof the bricks was raised to 200") F. or less and the surface bondsformed were silicates. it occurred to the applicants that they mightimprove the strength of the taconile nodules by utilizing this processbecause the surface of the taconite nodules is about 6-7% silica and ifthe spraying powder is limestone or dolomite, there is additionalavailable silica. However, the quality of silica is definitely low andthe formation of the bonds might be insufficient in number to materiallyglaze the surface of the taconite agglomeratc.

In the course of experimenting, with spraying heated taconiteagglomerates with powdered calcium carbonate (CaCOs, limestone) orcalcium-magnesium carbonates (CaMg(CO3)z, dolomite), it was discoveredthat a strong glaze was obtained at temperatures commencing at about2300 F., usually in the neighborhood of 24% F. It was at first assumedthat this glaze was a silicate glaze, but there just did not seem to besuflicient silica present to account for it. Moreover, the coloring didnot sug gest a silicate glaze, but rather indicated that the iron oxideplayed some part of the glaze. The experiments proved conclusively thatsomething was attained at a temperature around 2300 F. which was notthere no matter how long the temperature was maintained below 2100 F.

The bonds obtained are calcium ferrites or calcium magnesium ferritesand the reason for this is that the eutectic of calcium oxide,specifically Fezoa, and of calcium-magnesium oxide and iron oxide isslightly below .2400 F. Additionally, applicants nodule has calcium ironsilicate bonds and magnesium oxide silicate bonds where the raw materialis dolomitic limestone, which, however, are formed at lower temperaturesas disclosed in the copending application.

The fusion temperature of calcium oxide (C210) is in excess of 4000 F.The fusion temperature of iron oxide in the form F6203 is in excess of2400 F. The two, however, form a eutectic at approximately 1200" C. or2192 F. (Journal of the American Ceramic Society, vol. 30, No. 11, part2, page 24, Figure 46) to form calcium ferrite which has the formulaCaOFezOs. In the earliest,

simplest experiments performed by applicants, taconite nodules andpellets obtained from the three different companies evcloping theseprocesses on the Mesabi Range, were immersed in a slurry of water andpowdered limestone and then placed in an electric furnace. As the tornare passed about l50ll F. (the water having ciulicr upOriZcd) thecalcium carbonate (CaCOa) in the limestone converted to lime (CaO) thecarbon dioxide (fill) being released. As the temperature continued to bec alcium with or without iron formed a limited :r of silicates, eithercalcium silicate or calcium iron silicate. Also. the iron oxide in thelac-unite which had been in the form F0301 converted to FcaOs by pickingup :tdditimiul oxygen. As the temperature passed above the of iron oxide(FezOs) and calcium oxide (C(10), uumlwr or" calcium ferrite bonds wereformed. th: pellet shows a discolorization extending as .n is lit: of aninch into the surface of the pellet.

Equally satisfactory results were obtained by the use of dolomite.Dolomilc contains magnesium carbonates tall as calcium carbonates andthe bonds formed are managnesium ferrite bonds and magnesium iron rqcbonds, the latter forming at a lower temperature. The applicant has notbeen able to find in the literature the eutectic of calcium oxide,magnesium oxide and iron oxide, but it is reasonably apparent from thehardened nodule produced that it is below 2400 F. in short, cx rcrimentshows that the nodule treated with dolomite is as strong as the noduletreated with limestone.

in. the copending application, applicants had success uith maancsitc andmagnesia. At least to date, applicants have wtl] unable to make eitherof these work in their tucunitc agglomeratc surface hardening process.This is c plainaltlle by the fact that magnesium oxide and iron ide donot form a eutectic but enter into a solid solution. (Journal of theAmerican Ceramic Society. vol. 30, No. ll, page 27, Figure 58.) Thisinability of the pure magnesium carbonate to form a glaze on the surfaceof the taconite nodules at temperatures under 2506" F. confirmsapplicants conclusion that the bonds which they are forming are calciumferrite or calciummngnesium ferrite bonds and not merely silicates.

While applicants use calcium carbonates and magnesium carbonates becauselimestone and dolomite are cheap, the invention is not limited to thesetwo substances. The characteristics which the added substance mustpossess are ability to unite with iron oxide to form a strong bond, andability to enter into this union at a temperature in the neighborhood of2400 F.

The final important step in applicants process is determining how to getthe lime on the surface of the taconite agglomerates in commercialproduction. This is easily done in the case of the nodulizing processbecause the requisite temperature actually occurs in that process as thenodules move immediately under the flames of the final burner. The stepwhich applicants insert into the present process of nodulizing taconiteconsists in introducing lime, limestone, or dolomite, generally in agrain size of 200 mesh or smaller into the primary or secondary airsupplied for the hard flame burner 22 in a quantity sufficiently smallso that with an even dispersion of the flame over the tumbling nodulespassing under the hard flame burner 22, the heated powder compositiondoes not cover, but rather is sparsely dispersed over the nodules sothat only a limited fluxing occurs.

The calcium oxides and magnesium oxides do not volatilize even thoughthey are dispersed through the hard flame of the burner 22. The meltingpoint of either calcium oxide or magnesium oxide is in excess of 23G0(3., or in the neighborhood of 4500 F. The volatilization point is farhigher. In this respect, the coating agent is to be contrasted with thevarious chlorides, such as 0rdinary salt, which are used to vitrify orfuse or g aze ceramic wear. The chlorides volatilize in the flame heatand attack the surface of the ceramic and perform their function thereinitially in vapor form. Applicants problem is to spray thinly powderedmaterial (CaO or CaO-MgO mixtures) which at all nodulizing temperaturescan neither be fused nor vaporized. These powdered oxides, however, formwith the materials of the nodules eutectics which are slightly below thetemperature of the nodules adjacent the hard flame burner.

impinging powdered lime or dolomite on the tumbling nodules differs frommoving the nodules through an atmosphere of this powdered material onlyby way of degree. In U. S. Letters Patent 2,703,445, dated March 8,1955, the applicant Veale discloses a process of treating the surface ofordinary refractory brick with these same compounds for the purpose ofhardening the surfaces. In this application, the bricks are in a kilnfor a long period of time and the flow of air is comparatively slow,much slower than the flow of air arising from the main burner 18 of Fig.l of this application. Moreover, the refractories are fixed in size andare not growing.

In the present application, powdered calcium carbonate orcalcium-magnesium carbonate can be introduced to the chamber through theprimary or secondary air intake of the main burner which is not directedinto the nodules. Consequently, they would be blown toward the feed end26 of the kiln and there they would deposit on the smaller nodules atthat end and thereby weaken the nodules. At 2200 F., they would becomesoft and lose their shape. The composition of the present charge it notto be varied. Applicants introduction of the high eutectic flux is toincrease the hard surface characteristic of each nodule by addingceramic bonds at the surface only. Referring to Fig. l, applicants wishto create a zone between the dotted lines 36 and 38 where the hard flame22 is maintaining the nodules at their highest temperature, in whichzone there will be sufficient powdered calcium and magnesium oxide tocoat slightly each of the tumbling nodules so that a localized fluxingor a fluxing in situ occurs. This may not establish a completely glazedsurface, but rather a large number of glasslike bonds spaced from eachother to provide better access to the interior of the nodule. A fullyglazed surface is not desirable because it delays the penetration of theblast of a blast furnace. The ideal is the nodule as presently formedplus a partially interrupted glassy surface formed by calcium and/orcalcium-magnesium ferrites or any other calcium or magnesium compoundwhich is formed at a temperature below the maximum nodulizingtemperature of about 2400 F. to 2500 F.

The grain size of the material added may range from I00 mesh to muchfiner. This is dependent more upon the type of nozzle used and thevelocity of the air. The finer the particle, the smaller the surfacebond, and with equal weights of material, the more numerous the bonds.In general, as the examples disclosed below show, applicants utilize a200 mesh powder.

As for the quantity of surface reinforcing material that is introduced,this will depend firstly, upon how the material is applied to thenodules. This will depend in turn upon the type of burners used in thekiln. if a hard flame burner directed upon the tumbling nodules in acomparatively small area is utilized, the result will be thatpractically all of the powder (if not applied in excess) will beimpinged upon the nodules in such a fashion that all will be utilized ina fluxing action on the nodule surface. Very little will escape in thegases out the feed end of the kiln. On the other hand, if the kiln isutilizing only a soft flame burner, the nodules should be exposed topicking up this powdered reinforcing material for a greater period oftime, that is, over a longer distance of travel in the kiln. This meansthat the air in the upper part of the kiln should be more saturated withthis reinforcing powder and the result inevitably will be that more ofthe powder will be blown out the end of the kiln with the gases and morewill have to be introduced in order to properly coat the nodules.

Where the hard flame burner is used to disperse the powder, the quantitymay vary from about A; to A of a pound of the reinforcing material toone cubic foot of taconite charge. Inasmuch as the ground taconiteweighs 200 pounds per cubic foot, the formula may be stated as /2; tolbs. of reinforcing material to 200 lbs. of taconite fines.

A few examples will clarify the process.

Example I In a kiln having a main soft flame burner and an auxiliaryhard flame burner directed toward the tumbling nodules, with the feedend of the kiln being charged continuously with one cubic foot oftaconite fines per unit of time. applicants introduce into the primaryor secondary air line of the same burner, one-half pound of powderedlimestone having a grain size of 200 mesh or smaller. The powderedlimestone will form no part of the combustion step, but preferably itshould acquire a sufiiciently high temperature in the combustion spaceso that the CaCO converts to (1:10 plus CO2 with the result that whenCaO strikes the surface of the nodules, the bond between the calciumcarbonate and the iron oxide is substantially instantaneously formedbecause the two combining elements, while well below their own fusionpoints, are above the eutectic temperature. A good way is to add thepowdered limestone to a flowing stream of air which is to become theprimary air for the coal combustion. This air can be moved throughbaffles or other types of obstacles so as to disperse the limestoneparticles throughout the air. in turn, when this air picks up the coal,it disperses the coal uniformly throughout its volume so that at thepoint of combustion, there is a proper amount of air adjacent each coalparticle, and these limestone particles will, therefore, be dispersed atthe points of combustion between the air and the coal. As the nodulesflow through the zone 36-38, see Fig. l, the nodules in their final sizeacquire the surface bonds;

Example II With the same kiln conditions described under Example I,dolomite ground to a mesh of about 300 is introduced to the primary orsecondary air intake of the hard flame auxiliary burner 22 in an amountof pound per cubic foot of charge. In this case, the burner should bepositioned for a minimum distance between the nozzle and the tumblingsurface of the nodules. Every effort should be made to impinge all ofthe products of combustion upon the work, giving the secondary air aminimum chance of blowing away any of the particles. What actuallyhappens is that particles which escape from the direct blast are sweptupwardly where they are caught and carried on down the kiln by the lazyflame from the main burner.

Example 111 The conditions are identical with the first exampleexcepting that the kiln is equipped only with a main burner. In thiscase, it is desirable to maintain the mesh size of the reinforcingmaterial comparatively large so that there will be a tendency for thereinforcing material to fall by gravity upon the tumbling nodules. Here,applicants introduce the reinforcing material in a mesh size ofapproximately 100.

Example IV Burned iron ore agglomerates at room temperature are passedthrough a water slurry of powdered limestone, lime or dolomite. They arethen moved to an oven where they are heated to a temperature of about2400 F. The rate of heating is not important. The process can beperformed by moving the agglomerates on a conveyor through a kiln.

The process is applicable to the briquetting method in the way discussedin the copending application Serial No.

7 205,222, but magnesite and magnesia are not useful at the temperaturedisclosed in this application.

The process can be used for pelletizing by adding a stage of heating thepellets to a temperature of about 2400 F., and then reinforcing thesurface of the pellets during a tumbling operation by the same processdescribed for the nodulizing in the zone 3638 of Fig. 1. In thepelletizing process, the taconite is initially mixed with a charge of abinder such as bentonite and clay and pulverized coal. It is then movedinto a balling drum Where ingredients are agglomerated into balls thesize of a walnut. From this balling drum, the pellets move on a conveyor through a furnace which burns the coal to form what are reallysmall bricks. The temperature is not carried to a point above 2000" F.and strength is obtained primarily from the aluminum silicate bondswhich are obtained by the burning of the clay. However, there is presenton the surface of the pellets as they come from the burning furnacesuflicient F6203 to form with limestone or dolomite the type of ferritebonds disclosed by applicants herein and all that is necessary israising the temperature to about 2400 F. at the end of the kiln andproviding means to suspend the reinforcing particles in the air.Pelletizing has been performed in the same equipment as that used fornodulizing. A long rotary kiln is employed and the balling of a bindersuch as clay or bentonite with the taconite occurs at the feed end ofthis kiln. The balls quickly build up in size and thereafter movethrough the kiln at a substantailly constant size. During this movement,they are heated to a temperature of about 2000 F., the clay forming abond well below this heat. In order to adapt applicants process to thiskiln method of forming pellets, it will be necessary to raise thetemperature of the pellets still higher, that is, to a point some placenear 2400 F.

The process described in this application is applicable to agglomeratinghematite, which may become important for ore such as that found inVenezuela. The Venezuelan ores are highly pulverized and pelletizing isanticipated.

Applicants have attempted to verify the existence of the calcium oxide,magnesium oxide, iron oxide eutectic, but have been unable to find thephase diagram disclosing this. Professor Ralph E. Grimm, ResearchProfessor of the Department of Geology of the University of Illinois, aconsultant of the assignee of applicants, states that to his knowledgeno one has determined this, and that probably the better way ofexplaining the action of the dolomite is to state that in some instancesthe calcium ferrite bonds are replaced by magnesium ferrite bonds.Therefore, the explanation of calcium-magnesium oxide found in the aboveand in the claims that follow, should be interpreted in the light ofProfessor Grimms suggestion. The exact structure of thecalcium-magnesium ferrite is not clear.

Where the word nodules" is used in the claims, it includes pellets orany agglomerates that are being grown in a rotating kiln. Where the wordagglomerates" is used, it includes all usable sizes of iron ore massesor lumps, including bricks.

Having thus described applicants invention, what they claim is:

l. The method of strengthening iron ore agglomerates having substantialquantities of iron oxide in their surfaces which comprises contactingunder oxidizing conditions the surface of said agglomerates with smallerparticles composed essentially of an alkaline earth oxide selected fromthe group consisting of calcium oxide and calciummagnesium oxide withthe surfaces of the agglomeratcs at a temperature of at least the fusiontemperature of the eutectic of iron oxide and said alkaline earth oxideunder condtions permitting formation of a eutectic of the iron oxide andthe alkaline earth oxide.

2. The method of strengthening iron ore agglomerates having substantialquantities of iron oxide in their surlit faces which comprises the stepsof heating the agglomerates to a temperature above 2300 F., ofsuspending particles composed essentially of calcium oxide of a sizesuch as pass through a mesh screen in the fuel used for heating, and ofcontacting the calcium oxide particles therein with the surfaces of theagglomerates under oxidizing conditions permitting the formation of aeutectic of the iron oxide and the calcium oxide.

3. The method of strengthening iron ore agginmerates having substantialquantities of iron oxide in their surfaces which comprises the steps ofheating the agglomerates to a temperature above 2300 R, of suspendingparticles composed essentially of a mixture of calcium oxide andcalcium-magnesium oxide of a size such as pass through a 100 mesh screenin a flammable gas, and of contacting the mixture of calcium oxide andcalciummagnesium oxide particles therein with the surfaces of theagglomerates under oxidizing conditions permitting the formation of aeutectic of the iron oxide and the mixture of calcium oxide andcalcium-magnesium oxide.

4. The method of strengthening iron ore agglomerates having substantialquantities of iron oxide in their surfaces which comprises the steps ofheating the agglomerates to a temperature above 2300 F, and of impingingon the surface of the agglomerates under oxidizing conditions particlescomposed essentially of a material selected from the group consisting ofcalcium oxide and calcium-magnesium oxide under conditions permittingformation of a eutectic of the iron oxide and the material selected fromthe group consisting of calcium oxide and calcium-magnesium oxide.

5. The method of strengthening iron ore agglomerates having substantialquantities of iron oxide in their surfaces which comprises the steps ofheating the surface of the agglomerates to a temperature aboveapproximately 2300 F, and of impinging particles composed essentially ofcalcium oxide on the agglomerates under con-- ditions permittingformation of a eutectic of the iron oxide and the calcium oxide.

6. The method of strengthening iron ore agglomcrates whose surfacescontain in excess of 50 percent iron oxide which comprises the steps ofheating the agglomeratcs to a temperature above 2300 F., and of formingon the surfaces of the agglomerates under oxidizing conditions a glazecomposed essentially of the eutectic reaction product of a materialselected from the group consisting of calcium oxide andcalcium-magnesium oxide with the iron oxide of the agglomerates, saidglaze being formed by contacting particles composed essentially of saidmaterial with the surfaces of the heated agglomerates.

7. The method of surface strengthening taconite nodules havingsubstantial quantities of iron oxide in their surfaces which comprisesthe steps of raising the temperature of the surfaces of the nodules to atemperature of 2300 F, and of contacting under oxidizing conditionssmall particles composed essentially of a material selected from thegroup consisting of calcium oxide and calciummagnesium oxide with thesurfaces of the nodules to react with the iron oxide to form ferritebonds.

8. In the method of modulizing taconite agglomerates containing ironoxides in their surfaces wherein nodules are grown in a kiln by tumblingand heating under oxidizing conditions to a temperature in excess of2300" i- F., the step of passing the nodules at such temperaturesthrough an atmosphere having dispersed therein particles composedessentially of and selected from the group consisting of calcium oxideand calciurn -magnesium oxide so that the particles deposit on thesurfaces of the nodules to form ferrite bonds with the iron. oxide inthe taconite.

9. In the method of nodulizing taconite agglomerates containing ironoxides in their surfaces wherein nodules are grown in a kiln by tumblingand heating under oxidizing conditions to a temperature in excess of2300" F., the steps of heating solid particles composed essentially ofcalcium carbonate to a temperature approaching that of the eutectictemperature of calcium oxide with FezOs, and of suspending these heatedparticles in the path of the nodules at a point where the surfacetemperature of the nodules is in excess of said eutectic, so that theparticles deposit on the surfaces of the nodules and form with the ironoxide a bond.

10. In the method of strengthening taconite nodules containing ironoxides in their surfaces formed under oxidizing conditions in a rotarykiln at a maximum temperature which is above the fusion temperature ofthe eutectics of iron oxide and a material selected from the groupconsisting of calcium oxide and calcium-magnesium oxide, the step ofsuspending in the air of the kiln in the zone where the nodules attaintheir highest temperature solid particles composed essentially of atleast one of said materials as the nodules move through this zone ofsuspended particles, the particles contacting the surfaces of thenodules to form with the iron oxide in the taconite ferrite surfacebonds.

11. The method of surface strengthening iron ore agglomerates containingsubstantial quantities of iron oxides in their surfaces which are beingnodulized under oxidizing conditions in a rotary kiln having a zonewhere the agglomerates are heated above 2300 R, which comprises theadditional steps of suspending particles composed essentially of amaterial selected from the group consisting of calcium oxide andcalcium-magnesium oxide in a combustible mixture for the kiln burner,and of burning the mixture in contact with the nodules in that zone inthe kiln under conditions permitting formation on the surfaces of theagglomerates of a eutectic of the iron oxide and the material selectedfrom the group consisting essentially of calcium oxide andcalcium-magnesium oxide.

12. In the nodulizing of iron ore fines in a rotary kiln employing amain lazy flame burner and an auxiliary hard flame burner directedtoward the advancing nodules at a point where they have grown to theirfull size and are at a temperature above 2300 F. and contain substantialquantities of iron oxide in their surfaces, the step of introducing intothe combustion space of the auxiliary burner particles composedessentially of a material selected from the group of calcium oxide andcalciummagnesium oxide and having a size of less than mesh, underconditions permitting [ormation of a eutectic of the iron oxide and thematerial selected from the group consisting of calcium oxide andcalcium-magnesium oxide.

References Cited in the file of this patent UNITED STATES PATENTS792,449 Pohl June 13, 1905 822,929 Dellwik June 12, 1908 877,394Bergquist Ian. 21, 1908 1,847,596 Cavers et a1. Mar. 1, 1932 2,131,006Dean Sept. 20, 1938 2,137,049 Holmberg Nov. 15, 1938 2,184,078 Hyde Dec.19, 1939 2,243,785 Udy May 27, 1941 2,248,180 Mariarty July 8, 19412,416,550 Udy Feb. 25, 1947 2,416,551 Udy Feb. 25, 1947 2,605,179Lindemuth July 29, 1952 2,703,445 Veale Mar. 8, 1955 FOREIGN PATENTS9,007 Great Britain of 1905 575,053 Great Britain Jan. 31, 1946 642,339Great Britain Aug. 30, 1950 490,083 Canada Ian. 27, 1953 PublishingCorp, New York.

A. S. M. Review of Current Metal Literature, Ianu ary 1949, page 23.

7. THE METHOD OF SURFACE STRENGTHENING TACONITE NODULES HAVINGSUBSTANTIAL QUANTITIES OF IRON OXIDE IN THEIR SURFACES WHICH COMPRISESTHE STEP OF RAISING THE TEMPERATURE OF THE SURFACES OF THE NODUULES TO ATEMPERA2300* F., AND OF CONTACTING UNDER OXIDIZING CONDITIONS SMALLPARTICLES COMPOUND ESSENTIALLY OF A MATERIAL SELECTED FROM THE GROUPCONSISTING OF CALCIUM CXIDE AND CALCIUMMAGNESIUM OXIDE WITH THE SURFACESOF THE NODULES TO REACT WITH THE IRON OXIDE TO FROM FERRITE BONDS.